//给你二叉树的根节点 root ，返回其节点值的 锯齿形层序遍历 。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。 
//
// 
//
// 示例 1： 
// 
// 
//输入：root = [3,9,20,null,null,15,7]
//输出：[[3],[20,9],[15,7]]
// 
//
// 示例 2： 
//
// 
//输入：root = [1]
//输出：[[1]]
// 
//
// 示例 3： 
//
// 
//输入：root = []
//输出：[]
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//
// 
//
// 提示： 
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// 
// 树中节点数目在范围 [0, 2000] 内 
// -100 <= Node.val <= 100 
// 
//
// Related Topics 树 广度优先搜索 二叉树 👍 809 👎 0


//leetcode submit region begin(Prohibit modification and deletion)

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        boolean isOrder = true;
        if(root!=null){
            List<TreeNode> nodeList = new ArrayList<>();
            nodeList.add(root);
            while(nodeList!=null){
                List<Integer> levelList = new ArrayList<>();
                int size = nodeList.size();
                List<TreeNode> nodeList2 = new ArrayList<>();
                for(int i=0;i<size;i++){
                    TreeNode node = nodeList.get(i);
                    levelList.add(node.val);
                    TreeNode left = node.left;
                    TreeNode right = node.right;
                    if(left!=null){
                        nodeList2.add(left);
                    }
                    if(right!=null){
                        nodeList2.add(right);
                    }
                }
                if(levelList.size()>0){
                    if(!isOrder){
                        Collections.reverse(levelList);
                    }
                    result.add(levelList);
                    nodeList = nodeList2;
                }else{
                    nodeList = null;
                }
                isOrder = !isOrder;
            }
        }
        return result;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode() {}
    TreeNode(int val) { this.val = val; }
    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}
//leetcode submit region end(Prohibit modification and deletion)
